Inelastic electron tunneling spectroscopy in magnetic tunnel junctions with MgO(001) tunnel barrier

Fully, epitaxially-grown Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions were prepared by using electron beam evaporation under ultrahigh vacuum. The dynamic resistance curve and the inelastic electron tunnelling (IET) spectrum were measured simultaneously at 4.2 K by a lock-in technique using a homemade electrical circuit. Both curves show asymmetry with the bias. Peaks observed in the IET spectra for antiparallel (AP) magnetization around ±25 mV are considered to be due to magnon-assisted inelastic excitation. The strong intensity of the IET spectra in AP magnetization causes the decrease of tunneling magnetoresistance (TMR) ratio around the voltage of the peak position. The existence of spin-flip scattering sites at the interface between the bottom electrode and the MgO barrier can be attributed to the obvious shoulder observed around 150 mV for negative bias. These results imply that the strong asymmetry of the bias dependence of TMR could be improved by reducing the dislocations.